Approximately 1.2 million Americans suffer heart attacks each year. A large percentage of the heart attacks are caused by blood clots or thrombi which form within the coronary arteries. A thrombus is nature's way of stemming the loss of blood from its pipeline system by corking off an opening into the vascular tree. The biochemical process which results in thrombus formation is not fully understood. However, in simple terms, injury to the vascular wall releases chemicals which lead to conversion of soluble, circulating fibrinogen molecules into a polymeric structure of fibrin. The fibrin structure is insoluble and arranges itself into a three dimensional network of meshed strands which entraps red blood cells. The individual strands are approximately 0.2 microns in diameter and the mesh size is approximately 1 micron. Accordingly, five micron red blood cells are easily trapped within the three dimensional "net".
When a thrombus forms, it effectively stops the flow of blood through the zone of formation. If the thrombus extends across the interior diameter of an artery, it cuts off the flow of blood through the artery. If one of the coronary arteries is 100% thrombosed, the flow of-blood is stopped in that artery resulting in a shortage of oxygen carrying red blood cells to supply the muscle (myocardium), of the heart wall. Such a thrombosis is unnecessary to prevent loss of blood but can be undesirably triggered within an artery by damage to the arterial wall from atherosclerotic disease. Thus, the underlying disease of atherosclerosis may not cause acute-oxygen deficiency (ischemia) but can trigger, acute ischemia via induced thrombosis. Similarly, thrombosis of one of the carotid arteries can lead to stroke because of insufficient oxygen supply to vital nerve centers in the cranium. Strokes can also be caused by an embolus which has formed and then is dislodged from an upstream blood vessel or the heart itself. It is carried by flowing blood to a vessel supplying the brain where it lodges in the vessel, blocking the blood flow and triggering a stroke. These emboli may consist of thrombus, atheroslerotic plaque or a combination of the two. They are typically older and harder than fresh thrombus and resist the action of thrombolytic drugs.
Oxygen deficiency reduces or prohibits muscular activity, can cause chest pain (angina pectoris), and can lead to death of myocardium which permanently disables the heart to some extent. If the myocardial cell death is extensive, the heart will be unable to pump sufficient blood to supply the body's life sustaining needs. The extent of ischemia is affected by many factors, including the existence of collateral blood vessels and flow which can provide the necessary oxygen.
Coronary artery bypass graft (CABG) surgery is a surgical method for bypassing coronary arteries which, because of narrowing or obstruction, are unable to supply adequate oxygen to heart muscle. In recent years, direct administration of chemical lysing agents into the coronary arteries has shown to be of some benefit to patients who have thrombosed coronary arteries. In this procedure, a catheter is placed immediately in front of the blockage and a drip of streptokinase is positioned to be directed at the upstream side of the thrombus. Streptokinase is an enzyme which is able in time to dissolve the fibrin molecule. This procedure can take several hours and is not always successful in breaking up the thrombus. Furthermore, it can lead to downstream thrombus fragments (emboli) which can lead to blockage of small diameter branches. Auth, U.S. Pat. No. 4,646,736, discloses a thrombectomy device that permits rapid removal of an obstructive thrombus. However, the device is characterized by small catheter tip size and thus is unable to exert significant total force on clot masses. Also, a clot which is not in good position of purchase on a vessel wall in the "line of fire" of the rotating wire is not fibrinectomized. This is especially true of clots floating free in the blood stream, since it is virtually impossible to revolve within these clots in the absence of a constraint such as fingers.
Further disadvantages to this thrombectomy device include the difficulty of keeping the clot in the space above the wire during all degrees of rotation as the wire is moved sideways during rotation, which is sometimes necessary to sweep the arterial lumen. In fact, sweeping out an entire arterial lumen with a rotating wire is virtually impossible in all but the smallest, i.e., less than 1.5 mm diameter, arteries. An additional and serious possible disadvantage is that fragments of the clot may be embolized downstream.
Therefore, there has been a definite need for a thrombectomy device that can be more effective in sweeping arteries, in removing emboli that are free floating or not perfectly positioned, and in minimizing fragmentation of clots. These and other objects of the invention will become apparent from the following discussion of the invention